1. Field of the Invention
The present invention relates to a magnetron preferably applied to a device using a microwave such as a microwave oscillator.
2. Description of the Related Art
FIG. 17 is a longitudinally sectional view showing a magnetron disclosed in Patent Document 1. The magnetron shown in FIG. 17 mainly includes a magnetic yoke 4, an output part 9 provided in the upper part of the magnetic yoke 4 and a filter 11 provided in the lower part of the magnetic yoke 4. In the magnetic yoke 4, two annular permanent magnets 8A and 8B, an anode tubular member 10 and a cooling block 1 for covering the periphery of the anode tubular member 10 are accommodated. The filter 11 is provided with a choke coil 6 and a through capacitor 7.
The magnetic yoke 4 includes a main body part 4a with a form having one end (a lower end in FIG. 17) opened, the other end (an upper end in FIG. 17) closed and a hole (an illustration is omitted) opened at a central part and a cover part 4b for closing the opened end of the main body part 4a. In the central part of the cover part 4b, a hole is opened (an illustration is omitted) that is the same as the hole opened in the main body part 4.
The cooling block 1 is made of metal having a high thermal conductivity. In the cooling block, a cooling liquid circulating pipeline 2 for cooling liquid is formed. In the cooling liquid circulating pipeline 2, the cooling liquid is circulated. In the anode tubular member 10, anode vanes 12 are arranged in radial directions and a cavity resonator is formed by a space surrounded by the respectively adjacent anode vanes 12 and the anode tubular member 10. Further, in the central part of the anode tubular member 10, a cathode structural member 13 is disposed. A space surrounded by the cathode structural member 13 and the anode vanes 12 serves as a working space. On an upper end of the anode tubular member 10, an output side pole piece 14 is fixed and, to a lower end, an input side pole piece 15 is fixed.
The anode tubular member 10 is pressed from outside the annular permanent magnets 8A and 8B disposed in both upper and lower ends by the magnetic yoke 4. The annular permanent magnet 8B disposed in the lower side in of the drawing is a magnet of an input side and the annular permanent magnet 8A disposed in the upper side is a magnet of an output side.
The cooling block 1 serves to cool the anode tubular member 10 and its inner wall surface comes into closely contact with the outer wall surface of the anode tubular member 10 and its outer wall surface comes into closely contact with the inner wall surface of the magnetic yoke 4. A thermal diffusion compound 3 is applied to a contact part of the outer wall surface of the cooling block 1 and the inner wall surface of the magnetic yoke 4. Thus, if a gap should be formed in the contact part, a good thermally conductive state would be obtained and both the cooling block and the magnetic yoke could be secured to each other. In such a way, the cooling block 1 can cool the anode tubular member 10, the magnetic yoke 4, and the annular permanent magnets 8A and 8B and the filter 11 through the magnetic yoke 4.
When the usual magnetron is used, after the inner part of the magnetron is brought to a vacuum state, a desired electric power is applied to the cathode structural member 13 to discharge a thermo-electron and a dc high voltage is applied to a part between the anode vanes 12 and the cathode structural member 13. In the working space, a magnetic field is formed by the two annular permanent magnets 8 in the direction at right angles to the opposed direction of the cathode structural member 13 and the anode tubular member 10. The dc high voltage is applied to the part between the anode vanes 12 and the cathode structural member 13 so that electrons emitted from the cathode structural member 13 are pulled out toward the anode vanes 12. The electrons turn and circulate by an electric field and the magnetic filed in the working space to reach the anode vanes 12. Energy by the movement of the electrons is applied to the cavity resonator to contribute to the oscillation of the magnetron.                Patent Document 1: JP-A-3-297034        
However, the above-described usual magnetron has below-described problems.
Since the cooling block 1 comes into closely contact with the magnetic yoke 4, the cathode structural member 13 of the anode tubular member 10 is weak to an external impact as well as a vibration. In the cathode structural member 13, a filament for emitting electrons is provided. Since the filament has a quality very weak to the vibration or the impact and may be disconnected depending on the level of an external force or the vibration. When the filament is disconnected, the magnetron does not function.
Further, since the cooling block 1 is allowed to come into closely contact with the magnetic yoke 4, when the dimensional accuracy of them is not improved, an assembly is difficult. Even when these members can be assembled, if a gap between the cooling block 1 and the magnetic yoke 4 is large, an adhesion of the cooling block 1 and the magnetic yoke 4 is hardly improved even by applying the thermal diffusion compound 3.
Further, a corrosion (rust) may arise in a part where the cooling block 1 comes into closely contact with the magnetic yoke 4 depending on a material. For instance, when copper is used as a material of the cooling block, a difference in tendency of ionization becomes large between the magnetic yoke using iron and the cooling block, so that the magnetic yoke made of iron or (zinc) corrodes. In the liquid cooling type magnetron, since a dew condensation is apt to arise, the corrosion due to the difference in tendency of ionization is more accelerated. As examples that the difference in tendency of ionization is increased, copper and zinc, aluminum and iron and aluminum and zinc are exemplified as well as copper and iron.